Spectroscopic analysis is a broad field in which the composition and properties of a material in any phase, gas, liquid, solid, are determined from the electromagnetic spectra arising from the interaction (eg. absorption, luminescence, or emission) with energy. One aspect of spectrochemical analysis, known as spectroscopy, involves interaction of radiant energy with the material of interest. The particular methods used to study such matter-radiation interactions define many sub-fields of spectroscopy. One field in particular is known as absorption spectroscopy, in which the optical absorption spectra of liquid substances are measured. The absorption spectra is the distribution of light attenuation (due to absorbance) as a function of light wavelength. In a simple spectrophotometer the sample substance which is to be studied is placed in a transparent container, also known as a cuvette or sample cell. Electromagnetic radiation (light) of a known wavelength, λ, (ie. ultraviolet, infrared, visible, etc.) and intensity I is incident on one side of the cuvette. A detector, which measures the intensity of the exiting light, I is placed on the opposite side of the cuvette. The length that the light propagates through the sample is the distance d. Most standard UV/visible spectrophotometers utilize standard cuvettes which have 1 cm path lengths and normally hold 50 to 2000 μL of sample. For a sample consisting of a single homogeneous substance with a concentration c, the light transmitted through the sample will follow a relationship know as Beer's Law: A=ϵcl where A is the absorbance (also known as the optical density (OD) of the sample at wavelength λ where OD=the −log of the ratio of transmitted light to the incident light), ϵ is the absorptivity or extinction coefficient (normally at constant at a given wavelength), c is the concentration of the sample and l is the path length of light through the sample.
Spectroscopic measurements of solutions are widely used in various fields. Often the compound of interest in solution is highly concentrated. For example, certain biological samples, such as proteins, DNA or RNA are often isolated in concentrations that fall outside the linear range of the spectrophotometer when absorbance is measured. Therefore, dilution of the sample is often required to measure an absorbance value that falls within the linear range of the instrument. Frequently multiple dilutions of the sample are required which leads to both dilution errors and the removal of the sample diluted for any downstream application. It is, therefore, desirable to take existing samples with no knowledge of the possible concentration and measure the absorption of these samples without dilution.
Multiple sample cuvettes may solve the problem of repetitive sampling, however, this approach still requires the preparation of multiple sample cuvettes and removes some sample from further use. Furthermore, in most spectrophotometers the path length, l, is fixed.
Another approach to the dilution problem is to reduce the path length in making the absorbance measurement. By reducing the measurement path length, the sample volume can be reduced. Reduction of the path length also decreases the measured absorption proportionally to the path length decrease. For example, a reduction of path length from the standard 1 cm to a path length of 0.2 mm provides a virtual fifty-fold dilution. Therefore, the absorbance of more highly concentrated samples can be measure within the linear range of the instrument if the path length of the light travelling through the sample is decreased. There are several companies that manufacture cuvettes that while maintaining the 1 cm2 dimension of standard cuvettes decrease the path length through the sample by decreasing the interior volume. By decreasing the interior volume less sample is required and a more concentrated sample can be measured within the linear range of most standard spectrophotometers. While these low volume cuvettes enable the measurement of more concentrated samples the path length within these cuvettes is still fixed. If the sample concentration falls outside the linear range of the spectrophotometer the sample still may need to be diluted or another cuvette with an even smaller path length may be required before an accurate absorbance reading can be made.
The prior art also describes spectrophotometers and flow cells that are capable of measuring absorbance values of low volume samples. These devices are designed to utilize short path lengths for measuring absorbance so that only small amounts of sample are required. U.S. Pat. No. 4,643,580 to Gross et al. discloses a photometer head in which there is a housing for receiving and supporting small test volumes. A fiber optic transmitter and receiver are spaced within the housing so that a drop can be suspended between two ends.
U.S. Pat. No. 4,910,402 to McMillan discloses an apparatus in which a syringe drops liquid into the gap between two fixed fibers and an IR pulse from an LED laser is fed through the droplet. The output signal is analyzed as a function of the interaction of the radiation with the liquid of the drop.
U.S. Pat. No. 6,628,382 to Robertson describes an apparatus for performing spectrophotometric measurements on extremely small liquid samples in which a drop is held between two opposing surfaces by surface tension. The two surfaces can move relative to one another to keep the surface tension in a sample such that a spectrophotometric measurement by optical fibers can be made.
U.S. Pat. No. 6,747,740 to Leveille et al. describes a photometric measurement flow cell having measurement path lengths that can be adjusted down to less than 0.1 mm. The flow cell contains a stepped optical element which includes a stem portion that can be made to various lengths. The measurement path length can be adjusted by replacing one of the stepped elements of a particular length with another stepped element of a different length.
U.S. Pat. No. 6,188,474 to Dussault et al. describes a sample cell for use in spectroscopy that included two adjustable plates that enable a user to vary the cross sectional geometry of a sample cell flow path between two or more configurations.
U.S. Pat. No. 6,091,490 to Stellman et al. describes a fiber optic pipette coupled to a glass capillary for spectrophotometric measurements of small volume samples utilizing long path length capillary spectroscopy.
There are a series of patents assigned to Molecular Devices Corporation that describe a microplate reader capable of determining absorption measurements for multiple liquid samples in microtiter plates. Each well of the microtiter plate may provide for a different light path length based on the amount of sample solution in each well and the curvature of the meniscus of the solution in each well.
While some of these instruments provide the capability of varying the path length for measurement of highly concentrated low volume samples the applications described therein relate primarily to single path length and single wavelength measurements. Several of the instruments provide a limited number of path lengths and all are limited to path length larger than 0.2 mm. Furthermore, the devices and methods of the prior art do not provide for expanding the dynamic range of the spectrophotometer so that it is not necessary to adjust the concentration of the sample to fall within the linear range of absorbance detection of the instrument. To the extent that the prior art teaches shorter path lengths to determine the concentration of very concentrated samples or low volume samples the focus of these devices is to take a single absorbance reading at a single path length. As such the prior art references require that the path length be known with great accuracy so that an accurate concentration measurement can be made.
The present invention provides devices and methods that provide a variable path length spectrophotometer which dynamically adapts parameters in response to real time measurements via software control to expand the dynamic range of a conventionally spectrophotometer such that samples of almost any concentration can be measured without dilution or concentration of the original sample. Furthermore, certain methods of the present invention do not require that the path length be known to determine the concentration of samples. This and other objects and advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.